1. Field of the Invention
The invention relates to a novel method for growing silicon carbide single crystals by liquid phase deposition. More particularly, the invention is concerned with a method of growing silicon carbide single crystals by liquid phase deposition using a novel solution, which method makes it possible to reduce macro defects in the silicon carbide single crystals.
2. Description of the Related Art
Silicon carbide single crystals have excellent physical properties, such as considerably high thermal and chemical stability, high mechanical strength, high radiation hardness, higher breakdown voltage than that of Si, and high thermal conductivity. The silicon carbide single crystals to which a suitable impurity is added provide p-conductivity-type or n-conductivity-type semiconductors, which have a relatively large forbidden bandwidth (about 3.0 eV when a single crystal of 6H—SiC is used, about 3.3 eV when a single crystal of 4H—SiC is used). Accordingly, semiconductor devices using the silicon carbide single crystals can be used under high-temperature or high-frequency conditions, and offer a high withstand voltage and high resistance to harsh environments, which cannot be achieved by semiconductor devices using conventional semiconductor materials, such as silicon (Si) or gallium arsenide (GaAs). Thus, SiC (silicon carbide) has been increasingly expected as a next-generation semiconductor material.
Typical methods for growing silicon carbide single crystals include, for example, vapor phase deposition or vapor phase epitaxy (VPE), the Acheson method, and liquid phase deposition. Typical examples of the vapor phase deposition or VPE method include a sublimation process and chemical vapor deposition (CVD). In the sublimation process, various types of defects are likely to occur in the resultant crystal, and the crystal tends to be polycrystalline. The CVD method uses only gaseous sources as feed materials; therefore, the crystal formed by this method takes the form of a thin film. It is thus difficult to produce bulk single crystals by the CVD method. The Acheson method uses silica and coke as source materials, which are heated in an electric furnace; therefore, it is difficult or impossible for the resultant crystal to achieve high purity due to the presence of impurities, or the like, in the materials. In an example of method using liquid phase deposition, a silicon-containing alloy is dissolved into a melt in a graphite crucible, carbon is dissolved from the graphite crucible into the melt, and thus the melt is made a solution, so that a silicon carbide crystal layer is deposited and thus grown on a single crystal substrate placed in a low-temperature portion of the solution. Although silicon carbide single crystals are grown at a low rate by liquid phase deposition, in other words, liquid-phase production of silicon carbide single crystals suffers from a low growth rate, it is an advantageous method for obtaining bulk single crystals. Thus, various studies have been made in recent times, in an attempt to increase the growth rate in the growth of silicon carbide single crystals by liquid phase deposition, which does not suffer from the above-described problems as encountered in the vapor phase deposition and Acheson method.
In a method for producing a silicon carbide single crystal as described in JP-A-2000-264790, a source material containing at least one element selected from transition metals, Si and C (carbon) is dissolved into a melt (i.e., C (carbon) is dissolved into the melt which is a solvent containing the at least one element selected from transition metals and Si), with which a silicon carbide seed crystal in the form of a single crystal is brought into contact, and the solution is cooled into a condition where the temperature of the solution is lower than the liquidus line of the solution, so that a silicon carbide single crystal is deposited and grown on the seed crystal. While the transition metals listed by way of example in this publication are Fe, Co, Ni (which belong to the VIII group), Ti, Zr, Hf (which belong to the IVb group), V, Nb, Ta (which belong to the Vb group), and Cr, Mo, W (which belong to the VIb group), only the compositions of the materials containing Mo, Cr, or Co as a transition metal are specifically disclosed. In this publication, there are no disclosure of a method or means for measuring and evaluating the quality of the deposited single crystal, and no description of macro defects formed on a growth surface of the grown crystal.
JP-A-2004-2173 discloses a melt of an alloy containing Si, C and M (M: Mn or Ti) in which, where the atomic ratio of Si and M is represented by Si1-XMX, 0.1≦X≦0.7 when M is Mn, and 0.1≦X≦0.25 when M is Ti. The melt does not contain undissolved C. C is dissolved into the melt from a graphite crucible. In a method for producing silicon carbide single crystal as described in JP-A-2004-2173, a substrate of a silicon carbide seed crystal is dipped into the solution, and the alloy melt around the seed crystal substrate is supercooled so that the solution is supersaturated with silicon carbide, whereby a silicon carbide single crystal is grown on the seed crystal substrate. With regard to the method for producing a silicon carbide single crystal as described in JP-A-2000-264790 identified above, it is stated in JP-A-2004-2173 that the silicon carbide produced by this method is likely to be polycrystalline because of the inclusion of carbon in the source material.
JP-A-2006-143555 discloses a melt (a solution containing C) of an alloy containing Si, C and M (M: Fe or Co) in which, where [M] is the molar concentration of M and [Si] is the molar concentration of Si, a value of [M]/([M]+[Si]) is equal to or larger than 0.2 and equal to or smaller than 0.7 when M is Fe, and is equal to or larger than 0.05 and equal to or smaller than 0.25 when M is Co. In a method for producing a silicon carbide single crystal as described in JP-A-2006-143555, a seed crystal substrate made of silicon carbide is dipped into the melt of the alloy (the solution containing C), and the alloy melt around the seed crystal substrate is supersaturated with silicon carbide, whereby a silicon carbide single crystal is grown on the seed crystal substrate. This publication, however, has no description of micro defects on a growth surface of the crystal.
JP-A-2007-76986 discloses a solution that contains a melt that includes Si, Ti, M (M: Co and/or Mn) as a solvent and C as a solute, and satisfies the relationships of 0.17≦y/x≦0.33 and 0.90≦(y+/x≦1.80 where the atomic ratio of Si, Ti and M is represented by Six TiyMz, and a solution that contains Si, Ti, M (M: Al) and C, and satisfies the relationships of 0.17≦y/x≦0.33 and 0.33≦(y+z)/x≦0.60 where the atomic ratio of Si, Ti and M is represented by SixTiyMz. In a method for producing a silicon carbide single crystal as described in JP-A-2007-76986, a seed crystal substrate for use in the growth of silicon carbide is brought into contact with the former solution or the latter solution as indicated above, and the solution around the seed crystal substrate is supercooled so as to be supersaturated with silicon carbide dissolved in the solution, whereby a silicon carbide single crystal is grown on the seed crystal substrate. This publication, however, has no description of macro defects on a growth surface of the crystal.
As described above, there is no description of macro defects on growth surfaces of the crystals, in the above-identified publications describing the methods for growing silicon carbide bulk single crystals by liquid phase deposition, and it has been difficult to achieve an improvement in the morphology of surfaces of crystal growth layers.